Orthogonal Frequency Division Multiplexing (OFDM) is a transmission scheme which spreads the signal to be transmitted over a number of separate orthogonal sub-carriers. OFDM transmission has several advantages including high spectrum efficiency and resistance to multi-path interference. Accordingly, OFDM is a desirable transmission scheme for many wireless transmission applications. However, OFDM waveforms can have large peak-to-average power ratios (PAPR's). Large peaks cause problems because the power amplifiers used in the transmitter become less efficient as the peak-to-average power ratio (PAPR) of the RF signal increases. In addition, the finite dynamic range of the DAC's in the transmission system place limits on the PAPR. As a result, it is desirable to limit the PAPR to allow for more efficient design of the transmitter. This process is referred to as crest factor reduction (CFR).
CFR can be accomplished in various manners. The direct method is to clip peaks exceeding a specified level. This has the effect of moving the constellation elements of the transmitted data symbols from their assigned positions in the modulation scheme. The difference between the actual and assigned positions in the IQ space is referred to as the “constellation error” or “error vector magnitude” (EVM). The allowable relative constellation RMS error, averaged over sub-carriers, frames, and packets, is dependent on the rate modulation (where the most difficult specification is −31.0 dB for the ¾ rate 64-QAM). Clipping tends to distribute the constellation error over all the sub-carriers including null sub-carriers and pilot sub-carriers (used for frequency synchronization). In general, it is beneficial for the receiver to have accurate pilot information to minimize the bit error rate (BER). Although the constellation error is specified, the primary goal is to minimize the BER.
Other methods for CFR attempt to randomize the phase of the sub-carriers. These include “partial transmit sequence” (PTS) and “selective mapping” (SLM). The sub-carriers are multiplied by a set of different phase vectors producing a set of potential time sequences. The time sequence with the lowest PAPR is transmitted. Information regarding the phase vector used must be sent to the receiver to allow demodulation.
Another class of methods for CFR uses some of the data carriers as peak reducers. This includes “tone reservation.” Once a peak is detected in the time domain, the phase of the reserved sub-carriers are selected to reduce the peak. This results in a lost of bandwidth because less data sub-carriers are available. Information regarding the active data sub-carriers must be sent to the receiver.
Another method for CFR is to alter the constellation so that elements are not unique. This is referred to as “constellation extension.” Elements are mapped so that dIQ=−dIQ. The downside of this approach is that one bit is lost in the constellation mapping which reduces the throughput for the QPSK, 16-QAM, and 64-QAM to ½, ¾, and ⅚ of the original value, respectively. In addition, the Gray code mapping specified in the standard would have to be abandoned.
Accordingly, a need presently exists for a system and method to reduce PAPRs in an OFDM transmitter while avoiding the above noted problems of existing CFR approaches.